By Juliet Eilperin and Anthony Faiola Washington Post Staff WriterThursday, December 17, 2009; 7:01 AMCOPENHAGEN — The United States on Thursday announced it would help build a $100 billion annual fund by 2020 to help poor countries cope with climate change, but said its commitment depended on whether the nations gathered here could reach a substantive pact that includes “transparency” on tracking emissions cuts.

In the absence of a comprehensive pact, Clinton said, the United States would take its long-term financial pledge off the table. She did not specify how much the U.S. would contribute to the fund if a substantive agreement was reached. The European Union has also committed to building a longterm, $100 billion fund, while Japan has committed $15 billion in short-term funding to poor countries over the next three years if an agreement is reached.

Clinton’s announcement could help break a logjam that has stymied the talks for days, since developing countries have insisted they need to know how the industrialized world will help them adapt to climate change and curb their own emissions before signing off on a deal.

The $100 billion annual fund endorsed by Clinton would help poorer countries switch to less environmentally harmful forms of energy production and prepare for the impacts of rising seas and warmer global temperatures. Clinton did not detail how much the U.S. would contribute to that fund. She said there were a number of financing options under consideration, but would not provide details.

“$100 billion is a lot,” Clinton said. “It can have tangible effects.”

Clinton, however, warned that a failure to reach agreement in Copenhagen — a deal has been anticipated Friday when leaders including President Obama enter the final stage of talks — would put that pledge in jeopardy. Officials from the United States and other developed countries have consistently said they cannot accept an agreement that does not include pledged emission reductions from major developing countries, as well as a method of verifying those cuts. Such language is essential to U.S. senators, who have yet to pass climate legislation and would have to ratify any future climate treaty.